The invention relates to a method of preparing printed circuit boards, in which contact strips or electrical connection pads or conductive strips which cross one another with the interposition of an insulating layer are formed on a substrate of insulating material. The invention furthermore relates to a circuit board prepared by this method.
U.S. abandoned patent application Ser. No. 802,104 discloses a keyboard for ultrasonic and infrared remote control devices for electronic entertainment apparatus and for the operation of projectors as well as small electronic calculators, which consist essentially of a circuit board with conductor strips and switch contact surfaces and inexpensive pushbutton switches which are in electrical working connection with the switch contact surfaces of the printed circuit board.
For the improvement of the overall construction and for the simplification of the component parts, the circuit board of insulating material is provided with crossing conductor strips, the intersections between the first and second and additional superimposed conductor strips having electrically insulating layers. These electrically insulating layers can in a preferred manner consist of an insulating varnish. The advantage of such a keyboard is that, even in the case of a great number of required switch contact surfaces and conductor strips, only one circuit board is needed, with contact strips and switch contact surfaces applied to only one side. Furthermore, the special technique of the crossing conductor strips permits a circuit board of a small surface area, permitting the construction of handy, relatively small remote control apparatus.
It has been found, however, that especially where the conductor strips are close together, a migration of silver from one conductor strip of positive potential to an adjacent conductor strip of negative potential occurs, so that, especially in an environment of high atmospheric humidity, measurable short circuits can develop.
Attempts have been made to obviate this disadvantage by making the conductor strips only of silver, but making the switch points of a resistance coating of varnish and carbon black, referred to hereinafter as conductive varnish. Nevertheless, the problem mentioned above has not been solved in this manner, since the hard paper board used as the substrate absorbs moisture, and the silver of the conductor strips diffuses into the hard paper, and in this hard paper a galvanic migration of the silver ions occurs, so that, after a certain time of operation, the same undesirable situation develops.